1. Field of the Invention
The invention relates to a method and a device making it possible to protect against the copying of digital data stored on an information carrier.
2. Description of the Related Art
A possibility inherent in digital data is that they can be copied without appreciable loss of quality since copying consists in transmitting a series of “1”s and “0”s from the source to the recorder. The greatest number of errors which may occur when copying can be countered by using error correction methods. Thus, when an information carrier contains digital data, it is in principle relatively simple to record the content of the information carrier identically on a recordable carrier.
Numerous types and kinds of information carriers are used to store information of all sorts in digital form. For example, a magnetic tape, a recordable or non-recordable optical disk (CD, CD-R, CD-RW, DVD, DVD-R, magneto-optical disk, etc., respectively standing for Compact Disk, CD-Recordable, CD-Read Write, Digital Versatile Disk, DVD-Recordable) can store audio and/or video information in digital form.
In order to better safeguard for example the interests of the authors of stored information or those of producers of prerecorded information carriers, it is desirable to limit the possibilities of freely and simply copying the digital data. Various mechanisms and possibilities currently exist for protecting digital data against illegitimate copying.
In a known manner, the digital data can be encrypted when they are stored on the information carrier. Encryption makes it possible to limit the use of the digital data to the holder of a public or private deciphering key. Encryption is for example used in the protection of data on DVDs, optical disks used to store video data in digital form. Thus, a DVD player requires an appropriate key in order to decrypt the data read from the DVD.
One way of protecting digital data against copying consists in furnishing them with a watermark, that is to say with auxiliary data attached to the digital data. The watermark must be unmodifiable and non-erasable. The playing of the data is carried out with the aid of a public key which identifies the watermark. The public key is a code well known to the public, or more precisely contained in most players of information carriers. Should the watermarked digital data be copied, a private key is required in order to put the watermark back in place on the copy, failing which the copy becomes illegal devoid as it is of watermark. The private key is held by the author or the producer of the information thus watermarked. The digital data copied without watermark are no longer played by the player since the latter does not identify any watermark where it ought to find one. Thus, the watermark precludes copying without the private key. If a copy is necessary then the recorder must build in this private key.
Watermarking does not prevent the copying of the digital data by an analog route, that is to say copying which would firstly require conversion of the digital data into an analog signal and which would take the analog signal as the source of the copy.
A known solution for preventing the copying of a digital carrier by an analog route and more particularly in the field of video and television consists in corrupting the analog signal in such a way that it can be used to display an image on the screen of a television set by way of an analog input of this television set, but so that the same signal cannot be used to make a copy with a video recorder. More precisely, an electronic circuit is employed to influence image synchronization parameters. These synchronization parameters are perceived differently by a television set and by a video recorder. This solution does not make it possible to prevent the digital copying of digital data.
Another solution for limiting digital copies of digital data consists in furnishing the latter with copy generation management information. In principle, this information item conveys the information item “never copy” for data which do not have the right to be copied and the information item “copy” or “copy number X” if the data are a first generation or X-th generation copy of an original. Thus, a recorder can, with the aid of these information items, ascertain whether the digital data to be copied have the right to be copied digitally and prevent copying if it is prohibited for the 2nd or (X+1th) generation. The copy generation management information item is updated with each copy. This manipulation of the copy generation management information item renders it vulnerable to falsification. Specifically, the copy generation management information item is at one stage of copying available as plaintext, that is to say in decrypted form. The manipulation also requires the digital recorder to be equipped accordingly. The copy generation management information item does not by itself make it possible to prevent copying by an analog route.